The present invention relates to a method for wrapping a stretched plastic film around the circumference of a load by rotating the load and the plastic film feed roll each about its axis in order to unwind the stretchable plastic film from the feed roll and to wrap the stretched plastic film around the circumference of the load, and by braking the plastic film in order to stretch it between the braking point and the wrapping point.
The invention also relates to an apparatus for wrapping a stretched plastic film around the circumference of a load, the apparatus having means for supporting the load and for rotating it about its axis, bearing means for mounting the feed roll for the stretchable plastic film, and means for braking, between the feed roll and the load, the stretchable plastic film unwinding from the feed roll and the stretched plastic film being wrapped around the load.
Plastic film is often strengthened by stretching. Thereby the macromolecules of the film become parallel and the secondary bonding forces between them increase, greatly strengthening the plastic film in its stretch direction. An important measurement quantity in this case is the film stretch ratio, by which is meant the ratio between the lengths of the film when stretched and when unstretched, or in an ongoing process the speed ratio between the respective film portions. The film stretch ratio is determined according to the use of the film, and it is dependent on the properties of the plastic material of the film and on the stretching conditions, such as the temperature. A film of a suitable material may in advantageous conditions stretch up to 300%, in which case, for example, the strength of a polyethylene film in its stretch direction may increase approximately three-fold. When the width of the plastic film during stretching is maintained substantially constant, it follows from such an extent of stretching that large quantities of film material are saved.
When a stretched wrapping film is needed, it is possible to select a pre-stretched film of a suitable plastic material. Such a film has, however, the disadvantage that it will not adapt to the shape of the load during the wrapping process, will not form an elastically tensioned package, and will not necessarily by its stretch ratio and thickness be suitable for the specific product or product component concerned.
The disadvantage of a plastic wrapping film stretched at a constant force on the wrapping site is that the constant force emphasizes any cross-sectional variations present in the original film. At the same time, great variations in the stretch ratio are produced in the film. This is due to the fact that the thinnest areas in the film stretch more readily under the effect of the constant force and are thereby thinned to a relatively greater extent than are the thicker areas. Furthermore, it has proven difficult in a such a method to maintain constant the stretching force, e.g. the braking force, and thus variations of even other kinds have been produced in the stretch ratio.
By the use of a plastic film stretched on the site at a constant stretch ratio, a wrapping film on average of a suitable stretch ratio and thickness is obtained. U.S. Pat. No. 4,302,920 discloses the accomplishing of a constant stretch ratio between the feed roll and the load being wrapped, by means of a roller pair synchronized at different rotation speeds. However, in the method according to this patent, in which the stretching distance is between the rollers of the said roller pair, it has not been possible during the wrapping to take into account the film winding speed variations caused by the shape of the load to be packaged. Efforts to solve the problem have been made in U.S. Pat. No. 4,503,658 in which the tension of the film between the roller subsequent to the stretching distance and the load to be packaged is maintained constant, in which case the tension variation caused in the film by the shape of the load cannot pass to the area of the stretching distance to disturb the operation of the rollers operating at a constant stretch ratio. The measuring based on film tension is, however, deficient, since the elasticity of the film causes in the measurement a delay with respect to the rapid increases and decreases in the tension of the film, in which case the tension variations that the film is subjected to may, however, disturb the stretching.